linux_usbfs: Drop support for kernel versions earlier than 2.6.32

linux_usbfs: Drop support for kernel versions earlier than 2.6.32

The Linux backend plays lots of games to try and work with older
versions of the kernel that do not have certain features. Lets simplify
the backend by requiring at least 2.6.32 to use libusb. The only thing
remaining that still requires explicit version checking is the maximum
iso frame packet size.

Anything running 2.6.32 or later is sure to have a functional monotonic
clock, so this change also allows the removal of the get_timerfd_clock()
function from the backend as well as the check for a functional
monotonic clock during initialization.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>